969
FXUS63 KLSX 111949
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
249 PM CDT Mon May 11 2026

.KEY MESSAGES...

- There is a 30 to 60 percent chance of showers and thunderstorms
  Tuesday evening into overnight. A thunderstorm or two could
  contain gusty winds or small hail.

- Temperatures will warm to above average on Tuesday, then cool
  to near or slightly below average on Wednesday and Thursday.

- Warmer temperatures will return Friday into early next week
  along with opportunities for showers and thunderstorms at times.

&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 248 PM CDT Mon May 11 2026

An elongated surface high pressure center will gradually shift to
the southeast overnight tonight with portions of southeastern MO and
southwestern IL still seeing efficient radiational cooling with low
temperatures in the the upper 40s to around 50 F. Elsewhere, a
tightening surface pressure gradient and increasing southeasterly
winds will keep low temperatures warmer and in the 50s F. On
Tuesday, the Mid-Mississippi River Valley will be positioned in the
warm sector of an approaching cold front, with conditions favorable
for much warmer, above average temperatures in the upper 70s to mid-
80s F thanks to low-level southwesterly flow/WAA downsloped off the
Ozark Plateau and minimal clouds during peak insolation.

An upper-level trough tracking across the Upper Midwest and Great
Lakes late Tuesday will force a cold front southeastward into the
CWA during the evening into overnight. A narrow belt of mid-level
moist isentropic ascent near the front will promote development of
scattered showers and a few thunderstorms during the evening that
migrate southeastward overnight. However, coverage overall is most
confident along/east of the Mississippi River where HREF
probabilities of measurable rainfall are higher (40 to 60 percent)
closer to more vigorous trough-driven ascent passing to the
northeast. Strong deep-layer shear of 40 to 45 kt will be present,
but instability continues to be a limiting factor for severe
thunderstorms. With a lack of sustained Gulf of Mexico moisture
fetch including dewpoints only reaching the 50s F and poor mid-level
lapse rates owing to a warm layer/capping inversion, the latest HREF
only has 20 to 50 percent probabilities of 1000+ J/kg SBCAPE across
northeastern MO/west-central IL late Tuesday afternoon where locally
higher moisture is pooled along the cold front, quickly decreasing
with time and southeastward extent. It is also uncertain if there
will be surface-based thunderstorm initiation while instability is
greatest, if at all given the capping inversion. Therefore, the
predominant expectation is for high-based/elevated showers and
thunderstorms to develop across northeastern MO/west-central IL
during the evening with a few stronger thunderstorms capable of
gusty winds (plentiful sub-cloud dry air) and small hail, decreasing
in strength as they migrate eastward/southeastward overnight as
instability wanes.

Pfahler

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 248 PM CDT Mon May 11 2026

On Wednesday, post-frontal, low-level northerly flow and CAA beneath
upper-level northwesterly flow will provide cooler temperatures
across the Mid-Mississippi River Valley, but model guidance is in
agreement that the core of a seasonably cool post-frontal airmass
will remain across the Great Lakes and Upper Ohio Valley. Across the
CWA, high temperatures on Wednesday are forecast to range from the
upper 60s/around 70 F to the mid-70s F from east to west with a
gradient in 850-hPa temperatures. As a surface high pressure center
traverses the region late Wednesday through Thursday, CAA will
subside and WAA will eventually ensue, but it is uncertain if WAA
will begin early enough in the day Thursday to result in much change
in high temperatures from Wednesday.

Global model guidance vary in their depictions of the upper-level
flow pattern evolution late Thursday into early next week with key
differences in the amplitude and progression of a potential trough
across the Rocky Mountains to Desert Southwest, marking the
difference between quasi-zonal flow and more southwesterly flow.
Despite differences in exactly how this pattern transpires, there is
a general consensus for temperatures to return to above average
along with more quality Gulf of Mexico moisture return and passing
shortwave troughs leading to showers and thunderstorms at times.
Exactly how much temperatures warm will depend on the prevalence and
timing of clouds and precipitation, but there continues to be
concerns that the NBM rolling bias correction is warming
temperatures to the top of the ensemble model guidance distribution
and even beyond. This factor skews the current forecast high
temperatures based on the NBM to near daily records, which
confidence is not high in achieving with 850-hPa temperatures only
flirting with the 90th climatological percentile. Confidence is much
higher, based on ensemble model guidance, in high temperatures at
least largely in the 80s F through the period, potentially cooler if
showers and thunderstorms are timed during the day. In terms of
precipitation, 40 to 70 percent of ensemble model membership have
showers and thunderstorms Thursday night into Friday with a passing
shortwave trough and accompanying warm front and decaying cold
front. However, other details of this pattern governing
precipitation are difficult to discern at this point.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1236 PM CDT Mon May 11 2026

Dry and VFR flight conditions are expected to persist through
Tuesday afternoon at all terminals. Patchy river valley fog cannot
entirely be ruled out at KSUS tonight, but confidence is low in its
occurrence. Otherwise, winds will become southwesterly and
strengthen Tuesday morning as a surface high pressure center departs
and a cold front approaches from the northwest.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX